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-   -   Wall Roughness (http://www.cfd-online.com/Forums/floefd-floworks-flotherm/27162-wall-roughness.html)

LKF April 27, 2008 18:40

Wall Roughness
 
Has anybody had luck (or attempted) to simulate very rough walls in FLOWORKS? I'm currently trying to validate textbook calculations for Rz values in the order of 1E6 - 1E9 micro-inch and I am not getting results that correlate whatsoever.

Thanks for the insight/opinions.

LKF April 27, 2008 19:20

Re: Wall Roughness
 
Sorry, typo on the Rz values. I'm only looking at values up to about 3E6 micro-inch. Not 1E9!

John Parry April 28, 2008 07:26

Re: Wall Roughness
 
3E6 micro-inch is 3 inches. Can you elaborate on the application? How thick is the boundary layer?

John Parry, Research Manager, Flomerics Ltd.

LKF April 28, 2008 08:06

Re: Wall Roughness
 
Hello, thanks for the reply. Yes, 3E6 is 3 inches. Is this within the limits of the calculation scheme used by FLOWORKS to calculate wall shear stress and the resultant pressure drop along a flow domain?

The application I'm trying to model is loosely based on ambient airflow through mine shafts with shotcrete finished walls or just blasted/bored. It's large scale models (15 to 30 foot diameters) - I would anticipate bl thickness in the order of 6 inches to perhaps 18 inches. What scheme does FW use for bl approximation?

Additionally, where can I review the residual values after the calculations have finished iterating? The word 'residual' doesn't even show up in the FW help files when I search for it.

Thanks for the assistance,

LKF April 28, 2008 10:49

Re: Wall Roughness
 
I've continued to look into wall roughness effects. Not only are my results not lining up hand calcs, pressure drop along a 800 ft section (with fully developed flow) is not even varying for different Rz values.

Kelly Cordell-Morris May 8, 2008 10:22

Re: Wall Roughness
 
Do you have access to the validation examples in Floworks? In EFD Lab one of the validation examples is flow over smooth and rough plates. I have just quickly changed the parameters of one of these models and obtained a pressure drop profile. What parameters are you using to check for convergence? The drag coefficent would perhaps be one to use? Cd = F / ((0.5*rho*V^2)*A))

F=drag force, A=surface area, rho=fluid density, V=fluid velocity



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